Background of oysters

The Pacific oyster, a native species of Japan, was introduced to Australia in the 1940s. Pacific oysters attach to rocks and debris and can be found in estuaries and on sheltered rocky shores. They are found in the lower mid-tidal zone (0.5 to 2.2 m above low tide) but they also sometimes occur in the shallow sub-tidal zone. The shell is generally tear drop shaped but varies depending on growing conditions. The bottom shell is usually cupped. Overcrowding with other oysters during growth can cause distortions in the shell shape.

How big can oysters grow?

Pacific oysters can grow up to 450 mm in length however they are normally 55-100 mm. Pacific oysters are "filter feeders", which means they take in seawater from the surrounding environment and "filter" out any food particles present. Adult Pacific oysters are capable of clearing all the particles out of about 2-5 litres of water per hour. The main food for oysters is a wide variety of phytoplankton species (or microalgae).

How do they reproduce?

Pacific oysters begin life as males before becoming functional females. They have a simple reproductive system consisting of gonads (the creamy-white area of the oyster) which hold the gametes. In nature spawning is induced by a seasonal change and rise in water temperature; sexually mature oysters expel their sperm and eggs into the water, where the eggs are fertilised.

Life-Cycle

After spawning the fertilised egg hatches approximately 24 hours after fertilisation and results in the formation of free-floating larvae. Larvae are distributed to new areas of coastline by tides, currents and winds. After approximately 3 weeks in this freefloating phase the larvae enter the fixed stage of their life-cycle known as the "spat" or juvenile stage in which the individual is identical to the adult but around one fifth the size. In order for larvae to develop into oysters they must attach themselves to a suitable substrate and undergo metamorphosis into spat.